𧱠Retaining Wall Block Calculator
Blocks, caps, gravel, and drain pipe for your block retaining wall.
Wall Blocks (with waste)
How to Use This Calculator
Enter your wall length and height in feet, measured from the bottom of the buried base course to the top of the final block course (do not count the cap stones in the height). Pick your block size from the list and the length of the cap stones you plan to use. The calculator figures out the wall face area, divides by the face area of one block, and adds your waste allowance for cuts and breakage.
Measure the wall length along the face in feet. For a curved wall, run a tape measure along the curve at the base and add the waste option for curves.
Measure the wall height from the top of the compacted gravel base to the top of the last course. Remember the first course sits buried at least 6 inches, so include that buried depth.
Pick your block size. The calculator uses the block face area (length x height) to convert your wall square footage into a block count, then adds 5 percent waste by default.
Use the material list to buy: blocks, cap stones, base gravel in cubic yards, and perforated drain pipe in linear feet. Add drain gravel backfill behind the wall too.
The Formulas
The base gravel formula uses the standard 6-inch deep by 2-foot wide compacted stone base under the first course. Six inches is 0.5 feet, so the base volume is simply 0.5 x 2 x length in cubic feet, divided by 27 to get cubic yards, the unit gravel is sold in. The 12-inch drainage zone behind the wall uses roughly the same volume of gravel again, so plan on buying about double the base quantity if you are also backfilling with stone.
Block face area is the part of the block you see from the front: nominal length times nominal height. A 16x8 inch block is 128 square inches, which is 0.889 square feet. Note that block sizes are nominal, and you should verify the actual face area with your manufacturer before ordering.
Worked Example
A 40-foot long, 3-foot tall wall using standard 16x8 inch blocks with 5 percent waste. Wall face area = 40 x 3 = 120 square feet. Blocks = 120 / 0.889 = 135 blocks, times 1.05 = 142 blocks (rounded up). Cap stones with 18-inch caps = (40 x 12) / 18 = 26.7, so 27 caps. Base gravel = (0.5 x 2 x 40) / 27 = 1.48 cubic yards, so order 1.5 yards. Drain pipe = 40 linear feet. Buy an extra half yard of gravel for the backfill zone behind the wall.
Tips
Walls over 4 feet tall usually require a permit and an engineered design in most US jurisdictions, so check your local building department before you build. Use crushed angular stone (3/4 inch minus) for the base and backfill, never round river rock, because angular stone compacts and locks together. Set the perforated drain pipe with the holes facing down at the base of the wall, and always slope it to daylight. Glue caps with landscape block adhesive, and step back periodically during construction to keep courses level.
Estimates for planning only. Walls over 4 feet typically require engineering and permits; check your local building code.
Frequently Asked Questions
How do you calculate how many blocks you need for a retaining wall?
Multiply the wall length by the wall height to get the wall face area in square feet, then divide by the face area of one block. For example, a standard 16x8 inch block has a face of 0.889 square feet, so a 100 square foot wall needs about 113 blocks before waste. This calculator then adds 5 percent for cuts and breakage.
Do I need gravel behind a retaining wall?
Yes. Every segmental block wall needs a gravel base under the first course and drainage gravel behind the wall. The standard rule is a 6-inch deep, 2-foot wide compacted base of crushed stone, plus 12 inches of drainage gravel behind the wall. Without drainage, water pressure builds up and pushes the wall over, which is the number one cause of retaining wall failure.
How deep should the first course of block be buried?
The first course should sit below grade at least 1 inch for every 8 inches of wall height, or 6 inches minimum, whichever is greater. Burying the base course locks the wall in and prevents the bottom blocks from being pushed out. This calculator assumes you add the buried depth into your wall height input, so measure from the bottom of the base course.
What are cap stones and how many do I need?
Cap stones are the flat finishing units that sit on top of the final course. They protect the wall from water, give it a clean finished look, and are glued down with landscape adhesive. You need one cap per cap length along the wall: a 40-foot wall with 18-inch caps needs about 27 caps (480 inches divided by 18).
Do I need a drain pipe behind a retaining wall?
For any wall 3 feet or taller, yes. A perforated drain pipe (drain tile) set at the base of the wall, behind it in the gravel backfill, carries groundwater away before it can build pressure. This calculator gives you one linear foot of pipe per linear foot of wall, and you will want to daylight the pipe ends or tie them into a storm drain.